The present invention relates to a system and method for maintaining the stability, during braking, of a towed subvehicle in an articulated vehicle system, such as the semitrailer in a tractor-semitrailer system, and more particularly, to such a system and method for determining when to de-actuate or release the brakes, and whether to de-actuate or release the tractor brakes or the trailer brakes.
Brake control systems for all types of vehicles, including heavy-duty-tractor semitrailer articulated trucks, to improve the stopping and vehicle stability thereof, are now well known to those skilled in the art.
Brake systems of the anti-lock type, for all types of vehicles, are now well known in the prior art. Briefly, these systems operate to maintain vehicle stability (i.e., maintain an acceptable transverse co-efficient of friction of the braked wheels) by keeping the longitudinal slip of the braked wheels within a predetermined limit. Such control of the longitudinal slip usually requires modulating the braking forces on an individual wheel and/or individual axle basis, to permit at least some wheel rotation. Examples of prior art anti-lock brake ("ABS") systems may be seen by reference to U.S. Pat. Nos. 3,767,270; 3,854,556; 3,929,383; 3,966,267; 4,392,202; and 4,591,213, the disclosures of all of which are hereby incorporated by reference.
Brake systems for heavy-duty articulated vehicles, such as tractor-semitrailer trucks, are difficult to design, because the loading will vary substantially. For example, the loading on the tractor of a tractor-trailer may comprise the tractor only; the tractor with an empty or lightly loaded trailer, or the tractor with a heavily laden trailer. Furthermore, articulated tractor-trailer vehicles are, by their very nature, capable of exhibiting certain unstable dynamic behaviors, examples of which are "jackknife" and "trailer swing", each of which has its own characteristic cause, effect, and sequence of appropriate actions. Jackknife is sometimes referred to as "tractor brake caused jackknife", while trailer swing is sometimes referred to as "trailer brake caused jackknife". In other words, trailer instability may result from braking action at either the tractor or the trailer.